The ever expanding use of semiconductor light emitting devices has produced a highly competitive market for these devices. In this market, performance and price are often significant for providing product distinction among vendors.
One technique for improving the performance of a device is to increase the proportion of the generated light that is actually emitted from the device, and correspondingly reducing the amount of light that is trapped and eventually absorbed within the device. Using a reflector on the surface of the device that is not the light extraction surface is a common technique used to redirect light toward the light extraction surface, as is the use of reflectors on the sides of the device.
FIGS. 1A-1B illustrate the manufacture of example prior art light emitting devices with reflective sides, and FIG. 1C illustrates the emission of light from such a device.
In FIG. 1A, a substrate 110, commonly termed a submount, includes metal traces 120 for coupling an external power source to a light emitting element 150, typically via solder bumps 130. An underfill material 140 provides mechanical support to the light emitting element 150; the underfill material 140 may be reflective.
In this example, a wavelength conversion element 160 is situated above the light emitting surface 155 of the light emitting element 150. The wavelength conversion element absorbs some of the light emitted by the light emitting element 150 and emits light at a different wavelength. A mixture of the light emitted from the light emitting element 150 and the light emitted by the wavelength conversion element 160 exits the device from the light extraction surface 165.
In FIG. 1B, a reflective material 170 is applied to surround the device, so that light that strikes the sides/edges 162 of the wavelength conversion element 160 may be redirected toward the light extraction surface 165.
The substrate 110 may subsequently be sliced/diced to provide singulated devices. Optionally, a protective material, such as epoxy, may be molded or otherwise formed to encapsulate the light emitting device, either before or after the devices are singulated, and may be shaped to provide a particular optical effect.
FIG. 1C illustrates example emissions of light from the extraction surface 165 of the device. The surface 165 forms an interface between the wavelength conversion element 160 and the surrounding medium, and the indices of refraction of the wavelength conversion element and the surrounding medium will define a critical angle for light extraction, and this critical angle will define an escape cone 168 at each point on the surface 165. Light 101, 102 that strikes the surface at an angle relative to normal to the surface that is less than the critical angle (i.e. within the escape cone) will escape through the surface; light 106, 107 that strikes the surface at an angle greater than the critical angle will experience total internal reflection (TIR), and will be reflected away from the light extraction surface 165. As detailed further below, light that is totally internally reflected within the wavelength conversion element 160 is likely to be repeatedly totally internally reflected, and therefore highly likely to be absorbed within the light emitting device.